In recent years, due to higher density of optical disks, a shortest mark length that is a length of a shortest recording mark is approaching a limit of optical resolution and an increase in intersymbol interferences and degradation of SNRs (Signal Noise Rates) are becoming more noticeable. In consideration thereof, a PRML (Partial Response Maximum Likelihood) system is generally used as a signal processing method.
The PRML system is a technique that combines a partial response (PR) with maximum likelihood decoding (ML) and is a system which selects a most likely signal sequence from a reproduced waveform on the premise that a known intersymbol interference is to occur. Therefore, the PRML system is known for improved decoding performance as compared to conventional level judgment systems.
A signal reproduced from an optical disk is first subjected to partial response equalization using a waveform equalizer, a digital filter, and the like so as to have predetermined frequency characteristics and is subsequently decoded to corresponding binarized data by selecting a most likely state transition sequence from a trellis diagram using Viterbi decoding or the like. Generally, a value L which represents a likelihood of a state transition to a state Sn (where n denotes the number of states) until time k is defined by Equation (1) below.
                    L        =                              ∑                          i              =              0                        k                    ⁢                                          ⁢                                    (                                                y                  i                                -                                  E                  i                                            )                        2                                              (        1        )            
In Equation (1) above, yi denotes a value of a reproduced signal at time i and Ei denotes an expected ideal value of the reproduced signal. In a maximum likelihood decoding system, a state transition sequence for which the value L representing likelihood as calculated by Equation (1) is smallest is selected and decoded to corresponding binarized data (for example, refer to Patent Literature 1).
As the density of optical disks become higher, intersymbol interferences and SNR degradation become more problematic. Reproduction performance can be maintained by adopting a high-order PRML system as the PRML system. For example, when a 12 cm-diameter optical disk has a storage capacity of 25 GB per recording layer, favorable reproduction performance can be maintained by adopting a PR (1, 2, 2, 1) ML system. Meanwhile, when the storage capacity per layer is 33.3 GB, a PR (1, 2, 2, 2, 1) ML system must be adopted. In actuality, BDXLs which have a storage capacity of 33.4 GB per layer are already in practical use. BDXLs adopt the PR (1, 2, 2, 2, 1) ML system (for example, refer to Non Patent Literature 1).
In addition, the development of high-speed optical communication is being promoted in order to accommodate an explosion of information traffic in recent years. With the increase in information traffic, there is an apparent upward trend in inputted optical power and a physical limit of optical fibers where non-linear optical effects and thermal destruction occur is quickly approaching. In order to improve the communication efficiency of optical communication, there is a primary focus on improving the performance of transmitters and optical fibers which constitute transmission paths such as an optical wavelength multiplexing modulation system which uses light with a plurality of wavelengths and a dispersion-compensating fiber or an optical amplifier which compensate for distortion of an optical signal waveform due to dispersion caused by propagation characteristics of optical fibers.
With a PRML system used in optical disks as described above, a reproduced signal is equalized to frequency characteristics of a target PR class, and maximum likelihood decoding by a Viterbi algorithm or the like is performed on the PR-equalized reproduced signal according to a trellis diagram constructed based on the same PR class. For example, in the case of a BDXL with a storage capacity of 33.4 GB per layer, a PR (1, 2, 2, 2, 1) class is used.
FIG. 38 is a diagram showing frequency characteristics of an optical transfer function (OTF) of a BDXL and the PR (1, 2, 2, 2, 1) class. With a BDXL, high frequency noise is prevented from being greatly amplified by using the PR (1, 2, 2, 2, 1) class that is close to the frequency characteristics of the OTF. In this case, a reproduced signal corresponding to a recording mark of 1 channel bit is PR-equalized to a signal waveform of 5 channel bit width having an amplitude of (1, 2, 2, 2, 1). This is a state where an intersymbol interference of 5 channel bit width is created.
FIG. 39 is a diagram showing a trellis diagram that is applied to a Viterbi decoding circuit. As shown in FIG. 39, a trellis diagram used in maximum likelihood decoding has a constraint length of 5 in conformance with the intersymbol interference of 5 channel bit width and uses a BD RLL (1, 7) modulation system. In the trellis diagram shown in FIG. 39, the number of states at each time is 10.
FIG. 40 is a diagram showing frequency characteristics of an OTF of an optical disk with a recording line density that is twice of that of a BDXL and a PR (1, 2, 3, 4, 5, 6, 6, 6, 5, 4, 3, 2, 1) class that conforms to the OTF of the optical disk.
In this case, a reproduced signal corresponding to a recording mark of 1 channel bit is PR-equalized to a signal waveform of 11 channel bit width having an amplitude of (1, 2, 3, 4, 5, 6, 6, 6, 5, 4, 3, 2, 1). This is a state where an intersymbol interference of 13 channel bit width has occurred. A trellis diagram used in maximum likelihood decoding is to be constituted by the intersymbol interference of 13 channel bit width and a RLL (1, 7) modulation system of BD. As a result, the number of states increases to 466 and an enormous amount of processing becomes necessary.
FIG. 41 is a diagram showing a relationship between a width of an intersymbol interference and the number of states of a trellis diagram. FIG. 41 shows that as the width of the intersymbol interference increases, the number of states increases exponentially. Therefore, in order to simplify the trellis diagram, a length of a PR class must be reduced. However, reducing the length of a PR class increases high frequency gain, and causes a significant amplification of noise, a deterioration of SNR, and a degradation of decoding performance. Therefore, it is difficult to achieve a balance between decoding performance and simplification of a trellis diagram.
As described above, conventional PRML systems have a problem in that the amount of processing necessary for decoding increases as density of optical disks increase.
In addition, in optical communication, efficiency is achieved by the optical wavelength multiplexing modulation system, a dispersion-compensating fiber, or an optical amplifier as described earlier. Alternatively, in order to increase the amount of information that is transferred per wavelength of light, speed of optical modulation can be increased and a wider frequency band can be used to increase communication rate. However, in order to accurately receive an optical signal waveform modulated using a wide frequency band, an optical detector of a receiver must be improved. An optical detector converts a received optical signal waveform into an electric signal. Frequency bands that can be efficiently converted by an optical detector are limited.
Therefore, an intersymbol interference occurs during photoelectric conversion by an optical detector in which a signal transmitted at 1 channel bit width is detected as a waveform that has spread beyond the 1 channel bit width. The higher a channel frequency at an optical modulator of a transmitter, the wider the channel bit width of the intersymbol interference. Therefore, in a similar manner to optical disks, there is a problem in conventional optical communication in that when decoding digital information from a reception signal, the difficulty in achieving a balance between decoding performance and simplification of a trellis diagram prevents an improvement in communication rate or prevents a reduction in delay time. In addition, while sensitivity of an optical detector can be increased, this problematically requires an optical detector with a large size and greater power consumption.